This invention relates generally to the field of microfluidics, as applied to analysis of various biological and chemical compositions. More particularly, the invention provides methods and apparatus for carrying out analyses, using both imposed centrifugal forces and capillary forces resulting from the surface properties of the passageways in the apparatus
To determine the presence (or absence) of, or the amount of an analyte, such as glucose, albumin, or bacteria in bodily or other fluids, a reagent device is generally used to assist a technician performing the analysis. Such reagent devices contain one or more reagent areas at which the technician can apply the sample fluid and then compare the result to a standard. For example, a reagent strip is dipped into the sample fluid and the strip changes color, the intensity or type of color being compared with a standard reference color chart.
Preparation of such devices is difficult when the sample has a complex composition, as many bodily fluids do. The component to be identified or measured may have to be converted to a suitable form before it can be detected by a reagent to provide a characteristic color. Other components in the sample fluid may interfere with the desired reaction and they must be separated from the sample or their effect neutralized. Sometimes, the reagents are incompatible with each other. In other cases, the sample must be pre-treated to concentrate the analyte of interest. These and other problems make it difficult to provide in a single device the reagents and other components which are needed for a particular assay. The art contains many examples of devices intended to overcome such problems and to provide the ability to analyze a fluid sample.
A different approach is to carry out a sequence of steps which prepare and analyze a sample, but without requiring a technician to do so. One way of doing this is by preparing a device which does the desired processes automatically, but by keeping the reagents separated, is able to avoid the problems just discussed.
Carrying out analysis may involve receiving a sample, selecting a desired amount of the sample, diluting or washing the sample, separating it into components, and carrying out reactions with the sample or its components. If one were to carry out such steps in a laboratory on large samples, it would generally be necessary for a technician to manually perform the necessary steps or if automated, equipment would be needed to move the sample and its components and to introduce reagents, wash liquids, diluents and the like. However, it is typical of biological assays that the samples are small and therefore it follows that the processing steps must be carried out in very small equipment. Scaling down laboratory equipment to the size needed for samples of about 0.02 to 50 μL is not feasible and a different approach is used. Small vessels connected by micron-size passageways are made by creating such features in plastic or other suitable substrates and covering the resulting substrate with another layer. The vessels may contain reagents added to them before the covering layer is applied. The passageways may also be treated as desired to make them wettable or non-wettable by the sample to be tested. The sample, its components, or other fluids may move through such passageways by capillary action when the walls are wetted or they are prevented from moving when the fluids do not wet the walls of the passageway. Thus, the capillary passageways can either move fluids or prevent their movement as if a valve were present. Another method of moving fluids through capillary passageways is by centrifugal force, which overcomes the resistance of non-wettable walls. This simple description provides an overview of microfluidic devices. Specific applications are provided in many patents, some of which will be mentioned below.
An extended discussion of some of the principles used in arranging the vessels and passageways for various types of analyses is provided in U.S. Pat. No. 6,143,248. Additional examples of applications of those principles may be found in U.S. Pat. No. 6,063,589. The microfluidic devices described in those two patents were to be disposed in disc form and rotated on equipment capable of providing varying degrees of centrifugal force as needed to move fluids from one vessel to another. Generally, a sample would be supplied close to the center of rotation and gradually increasing rotational speeds would be used to move the sample, or portions of it, into vessels disposed further away from the center of rotation. The patents describe how specific amounts of samples can be isolated for analysis, how the samples can be mixed with other fluids for washing or other purposes, and how samples can be separated into their components.
Other patents describe the use of electrodes for moving fluids by electro-osmosis, such as U.S. Pat. No. 4,908,112. Caliper Technology Corporation has a portfolio of patent on microfluidic devices in which fluids are moved by electromotive propulsion. Representative examples are U.S. Pat. Nos. 5,942,443; 5,965,001; and 5,976,336.
In U.S. Pat. No. 5,141,868 capillary action is used to draw a sample into a cavity where measurements of the sample can be made by electrodes positioned in the sample cavity.
In U.S. patent application Ser. No. 10/082,415, microfluidic devices were disclosed which have advantages over those previously available. One such device employs multiple U-shaped capillary loops to provide metered amounts of sample liquid for subsequent analysis.
The present inventors have sought an improved method of metering multiple portions of a liquid sample having advantages over the device shown in U.S. patent application Ser. No. 10/082,415. Their solution to the problem is described in detail below.